Thermo-chemical joining of refractory metals



United States Patent This invention relates to a process for forming a bond or joint between metal pieces or sections and more particularly to a thermo-chemical process for accomplishing said bonding between sections of refractory metal.

There are many conventional methods of bonding two or more refractory metallic sections together. The prior art techniques include fusion welding which involves the melting of the base metal. Recrystallization of the base metal is generally attended by brittleness and an increase in the ductile-brittle transition temperature. Brazing and its variation, diffusion bonding, are other conventional methods. Both provide bonds with undesirable limitations. The heat resistance of a brazed joint is less than that of the base metal, and in some instances, extended heating at diffusion temperatures adversely affects the grain structure of the base metal. Soldering, another conventional process, is not satisfactory for structural applications due to the low melting temperature of the bond.

The present invention contemplates a novel process for bonding metals which, by employing low temperatures, does not affect the base metal grain structure.

In general, the present invention contemplates a process for forming a bond at a joint area defined between two or more metal sections that comprises providing a substantially non-oxidizing atmosphere at least surrounding said joint area, contacting at least said joint area with a mixture of gases including a vaporized metal salt and reducing gas therefor, heating at least said joint area and said mixture of gases to a temperature at which reduction of said salt to the metallic component thereof occurs, whereby said metallic component deposits as a bonding filler in said joint area.

Relative to thereducing gas, while hydrogen is preferred, others may be used.

Relative to the non-oxidizing atmosphere, the inert gases or mixtures thereof may be used. Examples are helium and argon. Also a vacuum may be employed.

Relative to the metal salt,'it is preferred that the salt be one which is a vapor at a temperature below that used in the deposition reaction. Additionally, it is preferred that the salt react at the deposition temperature to provide metallic ions and non-interfering easily disposable byproducts, these by-products preferably being in the gaseous state.

Preferably the salt is one of the refractory base metals. Salts of metals not the same as the base metal may be employed. The two base metal sections are preferably of the same metal. It is felt that the critical considera tions in the selection of the metals and the salts are the relative ductilities and coefiicients of expansion of the metals and metal salts involved. If the resultant residual stresses are tolerably low and the atomic bonding issatisfactory, the combination will provide an acceptable bond. For the same reasons, it is not necessary that both sections being joined be of the same metal. In the latter case,

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however, selection of the metal to be deposited is critical for it must be amenable to the two materials being joined. Atomic spacing and atomic size are two characteristics which must be considered.

The base metals which find particular utility in the inventive process are the high melting refractory metals, such as tungsten, molybdenum, titanium and hafnium. Generally speaking, the refractory metals may be considered as those having a melting point from about 2647 F. (the melting point of Ni) and above. Specific additional examples are nickel, iron and cobalt. Metals having an MP. above 3000 F. are particularly satisfactory. To a lesser extent this process exhibits utility with other metals such as aluminum, copper, tin and lead.

It is therefore an object of the presentinvention to provide a process for bonding together sections of such metals employing temperatures wherein the grain structure of the metals is substantially unaffected.

A more specific object of the present invention is to provide a highly effective method for bonding refractory metals.

Another object of the invention is to provide an efficient method for producing butt joints and lap or fillet joints between refractory metal members.

A more specific object is to provide such a deposition process in which the deposited metal and/or the base metal is refractory.

These and other objects of the invention will be apparent from the following detailed description and by reference to the accompanying drawings.

In the drawings:

FIGURE 1 is a flow sheet of the steps involved in the vapor deposition bonding of metal sections in accordance with this invention and FIGURE 2 is a side elevation showing the positioning of two sections of metal to be joined on a heater and the connection of a thermocouple thereto.

Referring to FIGURE 1, the base metal is designated bynumeral 10. As shown in FIGURE 2, the base metal may be inthe form of two metal sections 31. The heater 11 is essentially an alloy block to which heat is supplied by the torch 12 which may be a conventional welding torch with cooling water supplied thereto from the reservoir 13. The are struck between the torch and the alloy block heater 11 supplies heat by conduction through the alloy block to the base metal 10. The alloy block may be 1 x l x 4 inch solid tungstemmolybdenum alloy block. The base metal positioned onthis alloy block may be about three inches from the arc and in the same plane therewith.

The temperature of the base metal is controlled by controlling the amperage of the power supply to the torch. Of course, other heating means may be employed such as induction heating with suitable controls. Temperature can be measured by a thermocouple 14 fastened to or adjacent to the base metal as shown in FIGURE 2.

The alloy block heater 11 is grounded via the lead 15. Inert gas shielded tungsten arc torch 12 is supplied from the power source 40, shown here as a 400 ampere DC.

power supply. Shielding gas for the torch may be furnished through conduit 16 from the gas supply 17. Argon is a suitable gas to shield the electric are from contaminating gases and also assist in maintaining the chamber ride, are introduced from the reservoir 18, through conduit 19, into the gas mixer 20. A reducing gas, such as hydrogen, from the reservoir 21 is also fed through conduit 22 into gas mixer 20. The output of the gas mixer is connected through conduit 23 to the nozzle assembly 24, which may have single or multiple outlets. Of course, the mixture is maintained at a temperature above its condensation temperature and below its dissociation temperature. Vacuum pump 25 evacuates the chamber 26 through a filter 27. A pressure gauge 28 is connected to the chamber 26.

Example 1 Argon gas is supplied from reservoir 29 through conduit 30 to chamber 26 at the rate of 27 cubic feet per minute for three hours so as to assure that all oxygen is removed from said chamber. A constant one to two inches of positive water pressure is maintained in the chamber during the purge by means of the vacuum pump 25 connected to the outlet of the chamber. After purging, hydrogen from reservoir 21 is admitted for five minutes at a rate of about 2.5 liters per minute. The hydrogen passes through conduit 22, mixer 20, and conduit 23 into nozzle 24. The nozzle 24 directs the stream of hydrogen gas onto the surface of a tungsten base metal which is heated to approximately ll80-1200 F. The nozzle is positioned three-eighths of an inch above the surface of base metal. After the initial treatment with hydrogen, tungsten hexafluoride gas from the reservoir 18 is fed into the mixer 20 at a rate of about 0.5 liter per minute and is there mixed with hydrogen. This mixture of gases is then directed through the nozzle onto the surface of the base metal for a period of five minutes. During this time, the hydrogen gas reduces the tungsten hexafiuoride to metallic tungsten which is deposited on the base metal and the hydrogen combines with the fluoride to form hydrogen fluoride which is evacuated. Finally, the supply of tungsten hexafiuoride gas is stopped and after another period of five minutes, the hydrogen fiow is also terminated. Heating is then discontinued and the resultant structure is allowed to cool.

The base metal with a deposit thereon is polished and then etched with Murakamis reagent before examination. A hardness survey of the base metal and deposit shows that the deposited metal has a hardness of Rockwell C 48-50 and the base metal a hardness of Rockwell C 4847. j

A photomicrograph of the structure at 100 magnification clearly shows a tight adherent deposit on the tungsten base. It also indicates that the base metal grain structure is not affected by the deposition or by the temperature maintained during the process.

Example 2 In accordance with a second example, a butt joint was made employing the system of FIGURE 1. Two sections of base metal 31, as shown in FIGURE 2, are placed in bonding relation to each other on the heater 11. The joint is formed by depositing filler metal first on one side and then turning the specimen over and depositing the filler metal onto the other side. The base metal sections are A x /2 x .020 inch. The chamber is purged in the same manner as in Example 1 except that in this case the purging is limited to one hour. The base metal sections are maintained at about 1200-1265 F. and the deposition is conducted as in Example 1 for ten minutes, five minutes on each side. The resultant structure under similar examination shows a tight adherent bond between the filler metal and the two sections of the base metal and no adverse effect on the base metal grain structure. As in Example 1, the base metal is tungsten, the purging gas is argon, the reducing gas is hydrogen and the vaporized metallic salt is tungsten hexafluoride.

' the vaporized metal salt.

Lil Example 3 According to a third example, a butt joint was formed by simultaneously depositing metal on both sides of the joint by using a double nozzle. The double nozzle enables mixed gases to be directed onto both sides of the joint at the same time. All of the parameters for deposition are the same as noted in Example 2, except for the ,use of the double nozzle. The structure so produced provides results similar to those obtained in Example 2.

Example 4 A fourth run was made to form a lap or fillet joint using a single nozzle. In making the lap joint, two pieces of tungsten metal were overlapped and the filler metal was deposited against the end of one piece and onto the surface of the other. The single nozzle was positioned at about to the joint. The resultant structure also showed a tight, adherent bond between the filler metal and the base metal. No adverse effect on the base metal grain structure was observed.

While tungsten was used in the previous examples, other metals, and particularly refractory metals, can be employed. Also the concept is not limited to the use of hydrogen as the reducing gas or to tungsten hexafiuoride as It will be readily apparent to those skilled in the art that other reducing gases and metal salts may be employed according to the teaching of this invention with similar results.

While the .temperatures employed are preferably of from about 1000 to 1300 F., slightly lower temperatures may also be used as well as temperatures higher by 300 to 400 F. Nonetheless, a temperature in the range of l0OO to 1300 F. is preferred for use with refractory metals because it is more economical and there is less possibility of adverse effects on the base metal.

As other variations, helium may be used in place of argon. It is also feasible to employ a chamber filled with hydrogen gas and then introduce the metal salt vapors near the point of deposition or at the joint where it would automatically mix with the hydrogen rather than using a separate gas premixing chamber.

While specific embodiments of the present invention have been disclosed and described, other embodiments obvious to those skilled in the art from the teachings herein, are contemplated to be within the scope of the following claims.

What is claimed is:

1. A process for forming a bond at a joint area defined between two refractory metal sections that comprises providing a substantially non-oxidizing atmosphere at least surrounding said joint area, contacting at least said joint area with a mixture of gases consisting essentially of a vaporized salt of a metal that is capable of bonding to said sections and a reducing gas therefor, heating at least said joint area and said mixture of gases to a temperature at which reduction of said salt to the metallic component thereof occurs, whereby said metallic component deposits as a bonding filler in said joint area.

2. A process as defined in claim 1 further including the step of cooling said sections after suitable deposition of the bonding filler in said joint area.

3. A process as defined in claim 1 wherein said removal of oxygen is accomplished by purging said atmosphere with an inert gas.

4. A process as claimed in claim 3 wherein said inert gas is argon.

5. A process as claimed in claim 3 wherein said inert gas is helium.

6. A process as claimed in claim 3 wherein said inert gas is a mixture of inert gases.

7. A process as claimed in claim 1 wherein said removal of oxygen is accomplished by evacuation of the atmosphere surrounding said metal sections.

8. A process as defined in claim 1 wherein said refractory metal is tungsten.

9. A process as claimed in claim 8 wherein said reducing gas is hydrogen and said salt is tungsten hexafluon'de.

References Cited by the Examiner UNITED STATES PATENTS 2,130,879 9/1938 Dobke 29-504 X 2,249,723 7/1941 Orr 29-494 X 2,561,566 7/1951 Edson et a1. 29-494 X 10 2,674,790 4/1954 Edson et a1 29 504 X 2,674,791 4/1954 Edson et a1. 29-504 X 6 2,822,609 2/1958 Horvitz 29494 2,844,868 7/1958 Cline et a1. 29498 2,934,820 5/1960 Novak et al. 29-494 X 3,069,765 12/1962 Simpelaar 29494 X OTHER REFERENCES Procedures in Experimental Metallurgy by Seybolt and Burke, John Wiley and Sons, 1953, chapters 5, 6 and 11.

WHITMORE A. WILTZ, Primary Examiner.

JOHN F. CAMPBELL, Examiner.

P. COHEN, Assistant Examiner. 

1. A PROCESS FOR FORMING A BOND AT A JOINT AREA DEFINED BETWEEN TWO REFRACTORY METAL SECTIONS THAT COMPRISES PROVIDING A SUBSTANTIALLY NON-OXIDIZING ATMOSPHERE AT LEAST SURROUNDING SAID JOINT AREA, CONTACTING AT LEAST SAID JOINT AREA WITH A MIXTURE OF GASES CONSISTING ESSENTIALLY OF A VAPORIZED SALT OF A METAL THAT IS CAPABLE OF BONDING TO SAID SECTIONS AND A REDUCING GAS THEREFOR, HEATING AT LEAST SAID JOINT AREA AND SAID MIXTURE OF GASES TO A TEMPERATURE 